1. Field of the Invention
The present invention relates to a vehicle-mounted automotive alternator, and more particularly, to the number of blades of fans mounted on a rotor.
2. Description of the Related Art
FIG. 24 is a sectional view showing a construction of an ordinary automotive alternator. FIG. 25 is a perspective view of a rotor shown in FIG. 24.
The automotive alternator is arranged such that a Lundell-type rotor 7 is rotatably mounted in a case 3 composed of an aluminum front bracket 1 and an aluminum rear bracket 2 through a shaft 6, and a stator 8 is fastened to the inner wall surface of the case 3 so as to cover the inner circumference of the rotor 7.
The shaft 6 is rotatably supported by the front bracket 1 and the rear bracket 2. A pulley 4 is fastened to an end of the shaft 6 so as to transmit the rotational torque of an engine to the shaft 6 through a belt (not shown).
Slip rings 9 are fastened to the other end of the shaft 6 to supply current to the rotor 7 and a pair of brushes 10 are accommodated in a brush holder 11 disposed in the case 3 so as to be in sliding contact with the slip rings 9. A regulator 18 is adhered to a heat sink 17 fitted to the brush holder 11 to regulate the magnitude of AC voltage generated by the stator 8. Rectifiers 12, which are electrically connected to the stator 8, are mounted in the case 3 to rectify alternate current generated by the stator 8 to direct current.
The rotor 7 includes a rotor coil 13 for generating magnetic flux on passage of electric current and a pair of pole cores 20 and 21 disposed so as to cover the rotor coil 13, magnetic poles being formed in the pole cores 20 and 21 by magnetic flux generated in the rotor coil 13. The pair of poles cores 20 and 21 are made of iron and have cylindrical core base portions 22a and 23a and a plurality of claw-shaped magnetic poles 22 and 23 disposed radially externally on the outer circumferential perimeters of the core base portions 22a and 23a at even pitch, respectively, the end surfaces of the core base portions 22a and 23a are abutted against each other, and the pole cores 20 and 21 are fastened to the shaft 6 facing each other such that the claw-shaped magnetic poles 22 and 23 intermesh.
Fans 5, each having a plurality of blades 5c in the vicinity of the outer circumference thereof, are mounted on the end surfaces of the cylindrical core base portions 22a and 23a at both the ends of the rotor 7 to be driven in the axial direction thereof. Each fan 5 has a thin base sheet 5a and the plurality of blades 5c formed by cutting and raising the base sheet 5a. 
The stator 8 includes a stator core 15, and a stator coil 16 from which alternate current is generated by the change of magnetic flux from the rotor 7 as the rotor 7 rotates, the stator coil 16 being composed of a conductive wire wound around the stator core 15.
In the automotive alternator constructed in this manner, current is supplied from a battery (not shown) to the rotor coil 13 by means of the brushes 10 and the slip rings 9, and the magnetic flux is generated. The claw-shaped magnetic poles of one pole core 20 are magnetized to N polarities by the magnetic flux, and the claw-shaped magnetic poles 23 of the other pole core 21 are magnetized to S polarities. On the other hand, the rotational torque of the engine is transmitted to the shaft 6 by means of the belt and the pulley 4, and the rotor 7 is rotated. Thus, a rotating magnetic field is imparted to the stator coil 16, and electromotive force is generated in the stator coil 16. This alternating electromotive force is rectified to direct current by means of the rectifiers 12, its voltage is regulated by the regulator 18, and the battery is recharged.
In the automotive alternator, the rotor coil 13, the stator coil 16, the rectifiers 12, and the regulator 18 generate heat at all time while power is generated. To cool heat generated by the power generation, air intake vents 1a and 2a and air discharge vents lb and 2b are disposed in the front bracket 1 and the rear bracket 2.
As shown by an arrow in FIG. 24, at a rear-end, external air is sucked into the case 3 through the air intake vents 2a by means of the rotation of a fan 5 and cools the rectifiers 12 and the regulator 18, then cools the rear-end coil end of the stator coil 16 by being deflected centrifugally by the fan 5, and thereafter is discharged to the outside from the air discharge vents 2b. 
As shown by an arrow in FIG. 24, at a front-end, external air is sucked into the case 3 through the air intake vents 1a by the rotation of a fan 5, cools the front-end coil end of the stator coil 16 by being deflected centrifugally by the fan 5, and thereafter is discharged to the outside from the air discharge vents 1b. Further, cooling wind generated by a pressure difference between the front-end and the rear-end flows from the front-end to the rear-end through the inside of the rotor 7, and cools the rotor coil 13.
In general, automotive alternators, which are auxiliary machines mounted on vehicles, are required to have a performance to cope with the requirement for reducing noise outside vehicle and for making compartment quiet. The alternator includes a rotating member rotating at the large number of rotation at all times, which causes a problem of wind noise and magnetic noise. Further, the alternator, which is a heating member, is increasingly put in thermally hostile environments by the reduction of a space in which equipment are mounted. When, for example, the capacity of the fans 5 as a cooling means is increased to cope with the above problem, noise caused by the fans 5 may be increased. Further, when the cooling property of the alternator is reduced, an output of the alternator is decreased.
As a method of solving the above problem, Japanese Unexamined Patent Application Publication No. 11-220851 discloses to prevent an increase in noise by making the number of blades of fans 5 smaller than the number of claw-shaped magnetic poles. That is, the method prevents noise caused by the vibration of the claw-shaped magnetic poles when they rotate from being synchronized with noise caused by fan blades 5c. 
Incidentally, there is provided the even number of claw-shaped magnetic poles to alternately form N-poles and S-poles. Therefore, when the even number of fan blades 5c is used at any one of a front-end and a rear-end, they include the number of resonance which is resonated with the number of the claw-shaped magnetic poles. Particularly, in an automotive alternator whose frequency of rotation has a considerably wide range, it is a problem that the above method does not act as a perfect countermeasure for suppressing interference noise.
Accordingly, it is an object of the present invention, which was made to solve the above problems, to provide a small automotive alternator having a high output and excellent in a cost performance without reducing the cooling property thereof even at high environmental temperature.
In an automotive alternator according to the present invention including a case having a plurality of air intake vents disposed in the axial surface thereof and a plurality of air discharge vents disposed in the radial surface thereof, a rotor having a pair of pole cores including claw-shaped magnetic poles projecting radially externally from the outer circumferential perimeters thereof at even pitch, respectively, fastened to a shaft such that the claw-shaped magnetic poles intermesh and rotatably disposed in the case, the pair of pole cores having a pair of fans, which include a plurality of blades around the outer circumferences thereof, and being fastened to the axial end surfaces thereof; and a stator fastened to the case so as to cover the outer circumference of the rotor, and having a stator core including a plurality of slots formed around the inner circumference thereof facing the rotor and a stator winding accommodated in the slots, the slots are formed in an even number, each of the claw-shaped magnetic poles is formed in an even number, and the blades of the pair of fans are formed in the same odd number, respectively.
The number of the fan blades may be less than one-half the total number of the pair of claw-shaped magnetic poles.
Parts to be cooled are housed in the case at a rear-end and the amount of wind generated by a fan at a front-end may be larger than the amount of wind generated by a fan at the rear-end.
The outlet angle of the fan at the front-end may be larger than the outlet angle of the fan at the rear-end.
The outside diameter of the fan at the front-end may be larger than the outside diameter of the fan at the rear-end.
Further, the stator winding may include a plurality of windings in each of which one strand of wire is bent back outside the slots at the end surfaces of the stator core and wound into wave winding so as to alternately occupy an inner layer and an outer layer in a slot depth direction within the slots a predetermined number of slots apart, and the strand of wire bent back outside the slots at the end surfaces of the stator core may be arranged in a circumferential direction, thereby constituting coil end groups having approximately the same shape.
Further, the stator winding may include a plurality of windings in each of which one long strand of wire is bent back outside the slots at the end surfaces of the stator core and wound into wave winding so as to alternately occupy an inner layer and an outer layer in a slot depth direction within the slots a predetermined number of slots apart, and the turned portions of the strand of wire bent back outside the slots at the end surfaces of the stator core may be arranged in the circumferential direction, thereby constituting coil end groups, the coils ends having approximately the same shape at said front-end and at the rear-end.
Further, the number of the slots may be set to two in each pole and in each phase.